A microorganism is a living microscopic being such as bacteria, yeast and fungi, algae and protists. A microorganism can be unicellular or pluricellular. The larval stages of pluricellular organisms (metazoas) can also be the origin of biofilms.
The majority of microorganisms (pathogenic or non-pathogenic) have been studied up to the present in their “planktonic” form, free and isolated in a medium (cultivated in suspension or on a selective medium). In a natural medium outside of the laboratory the bacterial populations are found fixed on the support (“sessile” state) and developed in an organized community called a “biofilm”. This bacterial community is generally enclosed in a matrix of exopolysaccharides (EPS) limiting exchanges with the surrounding medium (A. Filloux, I. Vallet. Biofilm: “Mise en place and organisation d'une communauté bactérienne” (“Placing and Organization of a Bacterial Community”.) Medicine/Sciences 2003; 19: 77-83).
When a biofilm develops there is at first an adhesion of the bacteria on a support, then colonization of this support. When the bacteria multiply they rapidly form a film constituted of strata of cellular bodies secreting a sheath of exopolysaccharides that protects them against aggressions of the surrounding medium (Costerton et al. Bacterial Biofilms. Sciences 1999; 284-6). The kinetics of the formation of a biofilm can be subdivided into 5 stages:                Conditioning of the surface: The organic or mineral molecules present in the liquid phase will be absorbed on the surface in order to form a “conditioning film”.        Adherence or reversible adhesion: The microorganisms present approach the surfaces by gravimetry, Brownian movements or by chemotaxis if they possess flagella. During the course of this first fixation stage, causing only purely physical phenomena and weak physico-chemical interactions to occur, the microorganisms can still be readily detached.        Adhesion: This slower stage caused interactions with stronger energy to occur as well as the microbial metabolism and the cellular appendages of the microorganism (flagellae, pili, etc.). Adhesion is an active and specific phenomenon. The first colonizers will attach themselves in an irreversible manner to the surface in particular by the synthesis of exopolysaccharides. This process is relatively slow and is a function of environmental factors and of the microorganisms present.        The maturation of the biofilm (development and colonization of the surface): After having adhered to a surface the bacteria multiply and regroup in order to form microcolonies surrounded by polymers. The matrix of polymers (or glycocalyx) will act like a “cement” and reinforce the association of the bacteria among themselves and with the surface in order to finally form a biofilm and attain a state of equilibrium. The biofilm generally develops in a tri-dimensional structure that constitutes a confinement site. This microenvironment will be the seat of numerous physiological and molecular modifications relative to the plantonic growth mode. The biofilm formed in this manner will occupy all the surface that is offered to it if the conditions permit it to do so. The maturation of the biofilm is generally correlated with the production of EPS even if certain species of microorganisms do not synthesize or if only few polymers can likewise adhere and form biofilms on the surfaces.        Detachment: Biofilms are structures in perpetual dynamic equilibrium and develop as a function of the support, of the microorganisms and of the environment this development can be expressed by the detachments of cells or of aggregates.        
This release of cells into the liquid medium can allow as a consequence the contamination of the other surfaces and is in general the cause of numerous recurring diseases in a medical environment (source of resistances).
The nature of biofilms is very varied—some are very rich in ExoPolySaccharide (EPS) and others are principally constituted of bacterial bodies.
In human health, biofilms are responsible for infections that are becoming more and more difficult to suppress: in the entire ORL sphere (auditory conduit, nasal membrane, conjunctiva of the eye, etc.), on the teeth (appearance of tartar, caries, etc.), on the bronchi, the lungs (in patients afflicted with mucoviscidosis, etc.), in the urogenital tract, etc.
Furthermore, they are the origin of the majority of nosocomial pathologies (more than 10,000 deaths per year) by forming on catheters or implants (cardiac valves, artificial hips, urinary probes, etc.) (J. W. Costerton, P. Stewart and E. P. Greenberg, Bacterial Biofilms “A common cause of persistent infections”. Science, vol. 284, pp. 1318-1322).
Biofilms are also present in refrigeration towers, responsible for infection by legionellas.
They also affect the agrofood industry on account of their implication in cases of food poisoning (formation during ruptures in the cold chain, development on cutting tools, crunching tools and on work surfaces).
Likewise, biofilms develop in pipes, causing, in particular, corrosion phenomena.
Biofilms also develop on the surface of immerged objects, such as, e.g., boat hulls, causing problems of fouling (dirtying of the surface of boat hulls due to the colonization of the hulls by various microorganisms).
It should be noted that bacteria are not alone in creating biofilms: Fungi, algae and Protozoa also organize into biofilms.
Biofilms are therefore omnipresent in numerous areas, presenting sanitary risks and causing relatively significant damage.
However, the development and the behavior of these biofilms remains poorly understood due to the fact of their complexity when being studied, although numerous methods for studying the development of biofilms have been implemented.
The methods for studying biofilms are still principally based on the colonization of pieces of glass or of plastic immerged in a culture medium contained in flasks under agitation in drying ovens in order to subsequently color them crystal violet or to observe them under a microscope.
There are other more complex detection methods such as, e.g., detections by Micro-balance with quartz crystal (Q-CMD, Quartz Crystal Microbalance with Dissipation Monitoring), detections by MTA (Mass Transport Analysis), by UFDR (Ultrasonic Frequency Domain Reflectometry), by PCR in situ (on functional gene Amo A), by FISH (hybridization in situ under fluorescence), by CLSM (Confocal Laser Scanning Microscopy), by PAS (Photo Acoustic Spectroscopy), etc.
Still other methods use particles/magnetic beads covered with lectin, or antibodies for isolating the bacteria responsible for the development of the biofilm, in order to then allow the characterization of these microorganisms by classic methods of immunoanalysis or by molecular biology (hybridization or PCR).
However, such methods have proven to be difficult to implement and remain relatively onerous. Furthermore, they do not allow a sufficiently probing teaching to be given about the behavior of the bacteria and therefore about the formation and development of biofilms. In fact, these methods do not allow the development of a biofilm to be followed, whether it is simply constituted of cellular bodies (Listeria type), EPS (exopolysaccharide) or an analogous matrix secreted by colonizing microorganisms (Pseudomonas type).
FR 2555316 discloses a process and an apparatus for determining the viscosity of a fluid medium, which process consists of immersing a conductive bead into the fluid medium, applying a rotating magnetic field substantially centered on the bead, which rotating field is such that the flow of the fluid in contact with the bead put in rotation remains laminar, and determining a magnitude connected with the couple exerted on the bead by virtue of the viscosity of the fluid medium. Thus, the bead, plunged in a viscous medium, undergoes a moment of braking proportional to the viscosity and assumes a rotation as a permanent speed whose period is also proportional to the viscosity of the liquid medium to be analyzed. The rotation of the bead can be visualized with the aid of diffraction discs obtained by lighting the bead with the aid of a laser beam along its axis of rotation.
However, such a process is only adapted for an implementation in a homogeneous viscous medium. A culture medium of bacteria is opalescent, cloudy and opaque. Therefore, this method does not allow a determination of the formation or lack of formation of biofilms in the culture medium.
JP 61-161436 discloses a method for measuring the viscosity of a non-Newtonian fluid based on the principle of magnetic attraction. The method consists of measuring the viscosity by means of the measurement of the displacement and the displacement rate of a magnetized bar under the effect of a magnetic field.
That method proposed allows the determination of the characteristics relative to the viscous fluid such as the viscosity. However, the method in question does not allow in any way a reproduction of the behavior of a microorganism such as a bacteria developing in the viscous fluid.